KPNO 4 Meter Mayall - Basic Parameters and Facts

This page describes the current 4 Meter configuration

• Design concepts for very wide field Prime Focus and Cassegrain configurations
• Additional facts relevant to possible use of the 4 Meter for large science programs

Location: 

Longitude: 111:35:59.6 W,  Latitude: 31:57:50.5 N,  Altitude: 2123.2m MSL

* Note: The Airmass=2.1 limit was determined for Mosaic3. For DESI, with its 1.5 degree field, it's advised to point only to targets of airmass < 2.0 with the lower shutter raised.

DESI:

The DESI focal plane is 840 mm in diameter, covering 3 degrees on the sky. It consists of 10 pie-shaped “petals”, each of which has 500 fiber actuators, 12 fixed “fiducial” fibers used to calibrate the locations of the actuators, and a CCD imager which is used to position the telescope to the desired position and ensure it is focused. The 500 fibers from each petal are wrapped in a 26 mm diameter cable 50 m long which delivers the light to one of the 10 spectrographs.

Each fiber subtends about 1.5 arcseconds on the sky and can be positioned to an accuracy of 0.05 arcsec. The positioners use two small motors to position a fiber within a patrol range 12 mm (171 arcsec) in diameter. They are typically spaced by about 10.5 mm (150 arcsec) to achieve full coverage of the focal plane. The positioning software stages the motions to avoid collisions between adjacent positioners.

Each spectrograph covers the 360 – 980 nm spectral range with a resolution between 3500 and 5000. Two dichroics are used to split the spectrum into three wavelength bands, each of which is dispersed by a grating and focused by a camera specifically optimized for that particular range of wavelengths to maximize the efficiency. The CCD detectors are also chosen for best performance in their wavelength range. Each CCD is cooled by a mechanical refrigerator to approximately -100 C to reduce the noise.

The 10 spectrographs are housed in a room temperature controlled to a precision of 0.1 C to avoid thermal expansion effects that would move the spectra on the detectors.

In between observations, a LED inside each spectrograph back-illuminates the fibers, which are imaged by a camera mounted in the Cassegrain Cage of the telescope. The actual position of each fiber is compared to the position predicted from the known coordinates of the target galaxies, and a series of small motions moves the fibers to the correct position.

Typical DESI exposures are 18 – 20 minutes long. After each exposure is done, the CCDs are read out, the telescope is moved to the next field, guide stars are acquired and the telescope is moved to the correct position, the fibers are back-illuminated, imaged, and tweaked to the target positions. These steps are done in parallel so DESI is able to start a new exposure within 2 minutes of completing the previous one.